As it is well known, an inkjet printhead is able to generate a plurality of dots on a print medium, the nozzles being the elements that are able to generate single dots on the print medium by ejecting ink drops. Typically, the printhead ejects the ink drops through the nozzles by rapidly heating a small volume of ink located in vaporization chambers with small electric heaters, such as thin film resistors. Heating the ink causes the ink to vaporize and be ejected from the nozzles (also known as “firing”).
An inkjet printer produces a printed image by printing a pattern of individual dots (or pixels) at specific locations of an array. These dot locations are defined by the pattern to be printed. In order to produce a printed image through a printer, the printhead is mounted on a carriage that is moved transversally to the print medium and the print medium is moved longitudinally, i.e. perpendicularly to the translation direction of the carriage and of the printhead; therefore, the printhead is designed for having a specific transversal translation direction. In most printers, in order to reduce the print time, the printhead prints, i.e. its nozzles eject ink drops, when the carriage moves in a first direction, e.g. from left to right, as well as when the carriage moves in a second direction opposite to the first direction, e.g. from right to left.
An ink supply, such as an ink reservoir, supplies ink to the nozzles and a control unit controls the ejection of ink drops from the nozzles, i.e. the firing of the nozzles, according to the patterns to be printed.
The nozzles of a printhead are usually grouped in one or more vertical print columns adjacent to each other in the transversal direction.
U.S. Pat. No. 6,478,396 discloses a printhead including a group of nozzles and a group of firing resistors corresponding to the group of nozzles. The printhead includes a programmable nozzle firing order controller configured to provide address generator control signals; various nozzle address sequences are provided based on a skipping approach.
U.S. Pat. No. 6,318,828 discloses a printhead assembly that controls the firing operations of the printhead. A detailed structural and functional description is provided of a printing system, a printhead assembly and a printhead.
Inkjet ejectors (nozzles) can be arranged in different layouts in the print column. As described in U.S. Pat. No. 5,907,331, activating an array of ejectors in their natural order may result in droplets emitted in neighbouring ejectors splashing against each other, thus resulting in undesirable print defects. A different order will ensure that an ejector to be activated is a number of ejectors away from the previous ejector that was activated.
As in practice it is difficult to manufacture a printhead where many nozzles would fire at the same time, it is common to divide each print column of a printhead into print groups of nozzles and to stagger the nozzles of each print group along the transversal direction and to fire only one nozzle per print group at the same time; usually a constant pitch is used. Since in each group the nozzles are located at different positions along the transversal direction, in order to produce a vertical line through the print column, it is necessary to address the nozzles sequentially, according to a suitable timing. Said timing depends among other things on the translation speed of the printhead in the transversal direction. In a printhead comprising groups of staggered nozzles, for each group a staggering width is defined corresponding to the number of nozzles in the group multiplied by the pitch; in other terms, the staggering width corresponds substantially to the distance between the first nozzle in the transversal direction and the last nozzle in the transversal direction. Therefore, a staggered nozzles printhead is associated to an intrinsic transversal printing resolution, i.e. its staggering width, which can be defined as its standard or normal transversal printing resolution.
From U.S. Pat. No. 6,669,330, it is known a method for printing through one staggered nozzles printhead with resolutions that differ from the standard resolution of the printhead. The speed in the transversal direction is changed with reference to a reference velocity, which the printhead is intended to be driven with, while preferably keeping the firing frequency of its nozzles unchanged. The firing order of the nozzles may or may not be changed.
The Applicant has considered the technical teaching of this document and has realized that each print group in the print column is divided vertically into a number N of adjacent and identical staggered sets each of a number M of nozzles, that only two firing orders are used for printing, that is to say the direct order e.g. ABC or ABCD and the reverse order e.g. CBA or DCBA, and that the possible resolutions that can be obtained are the following multiples of the standard resolution:                M i+1,        M i−1.        
The Applicant has understood that, in practice, according to the teaching of this document, in order to have many small multiples, e.g. 2 3 4 5 6 7 8 9 . . . , of the standard resolution, which would be desirable, M must be either 3 or 4; if M=3 the multiples are 2 4 5 7 8 . . . ; if M=4, the multiples are 3 5 7 9 . . . .
According to the teaching of this document, N nozzles fires at the same time in the same group of the same print column. As already said, the Applicant has noted that this would be difficult to realize and would require that the total number T of nozzles of the group of the print column be very small; for example, if N=2 and M=3 T=2×3=6, or, if N=2 and M=4 T=2×4=8. Anyway, according to the Applicant's knowledge and experience, in practical applications the print groups of a print column comprise at least 10 nozzles each, preferably more.
It is an object of the present invention to provide a method of addressing the nozzles of a staggered printhead whereby different print resolutions may be obtained with a single pass of the printhead without the limitations and constraints of the methods according to the prior art.
This object is achieved through the teaching of the present invention.